When "that doesn't work for us" doesn't work?

[NomsQualityStreets]

As a bit of background (posted about it before) my OH and DP(arents) are NC with each other.

Im currently on mat leave and usually make time to see DPs once a week for most of the day (5-6hours) when my DM is off work.
She usually texts me the days she's off and we go from there.
It's never weekends as that's usually our "family time" with OH and the DCs and we usually do stuff together.

So my problem is on weeks like this one where she's off on 2 weekdays DC1 is at nursery and on Saturday which technically we are free on but its not a great time for us as mentioned above.
I don't want to go into the reasons we can't come this week as it will just lead to tension and conflict as DM despises OH and it will be all "you never come at weekends" "you've got the whole weekend for family time" etc or just sulks/moodiness and I'm sure she's keeping a mental note of these things to pull out at some point.
But "sorry that doesn't work for us" doesn't sit with them and there's "why not, just tell me" "why can't you do it" etc and if I just say it on repeat its seen as very rude (different language so comes across a bit different).

How do I assert myself without being rude or leading to tension?

I always chicken out of saying "OH doesn't trust you with DCs" as I really don't want to face the conflict that would cause.

I cant believe you typed that and posted it. Why don't you go NC or even LC (low contact) with your parents, especially as it seems your OH's choice to go NC is completely understandable?! Your OH doesn't have a MiL problem, he has a DW problem - you!

You are the problem for your family, you know.

By family, I don't mean your so-called 'dear' parents- I mean the family you should have some loyalty to, and don't.

You're going to get the same answers again and again.

Answers on repeat 'seem very rude', and that is a problem apparently, but it's fine for your DP to utterly screw over your own husband and family and that's not 'rude' enough for you to take a stand?

I thought she would sulk/be in a mood over the fact I won't come down at the weekend but turns out she's fine. Similarly I sometimes think she will be ok with something and turns out that's not the case. - hmm, I bet that keeps you on your toes, eh?

Wonder why that is?

I think she should be grateful that she sees you every week, for practically the entire day too.

Just say 'sorry, we can't this weekend, I'll see you on your day off next week'

You just have to keep repeating it.

That doesn't work for us.

But why not?

That is not up for discussion.

But why can't we?

I'm not going to discuss that with you.

Keep on saying no, and meaning it, and if they refuse to let the subject drop then you either leave the room without further comment, or you tell them you're going to hang up the phone and then you do it.

You have to be strong, and be determined.

You can do this.

Don't deal with it by making excuses or inventing social obligations that don't exist: that just complicates the issue. Be clear, refuse to be drawn, and if all else fails, leave the conversation.